Please refer to FIG. 1, it shows the schematic circuit diagram of a traditional three-phase power factor correction converter of the prior art. In which, the three-phase power factor correction converter includes three filter capacitors Crt, Crs, and Cst, three boost inductors Lr, Ls, and Lt, six diodes Dr, Ds, Dt, Du, Dv, and Dw, and an output capacitor Cb. Although this apparatus has the advantage of having a relatively simple configuration, but the power factor of such an apparatus could only reach the highest value of around 0.93. Besides, the inductors of the above-mentioned apparatus, made of silicon steel, are relatively heavier and larger. Due to the aforementioned drawbacks, the passive power factor correction method employed in the above-mentioned circuit as shown in FIG. 1 has been replaced by an active power factor correction method employed in a different circuit of the three-phase power factor correction converter introduced hereinafter.
Please refer to FIG. 2, it shows the schematic circuit diagram of another three-phase power factor correction converter of the prior art. This apparatus is proposed to improve the aforementioned drawbacks of the three-phase power factor correction converter of the prior art. The differences between the apparatus of FIG. 1 and the apparatus of FIG. 2 are that an active switch S and a diode Db are included in the circuit of FIG. 2 so as to achieve a higher power factor and to lower the total harmonic distortion (THD) through the closing and opening of the switch S. However, there are several obvious drawbacks regarding the apparatus of FIG. 2 described as follows.
Firstly, there are relatively higher switching losses of the active switch S due to the reverse recovery time of the diode Db. When the output is a relatively higher voltage value, 800 VDC for example, the switching losses of the active switch S are even worse relatively. Secondly, the desired requirements of decreasing the sizes of the magnetic elements for the apparatus of FIG. 2 are infeasible since the switching frequency must be increased in order to do so and which will result in the relatively higher switching losses. Thirdly, the desired requirement of having a THD less than 5% could not be achieved by the apparatus as shown in FIG. 2.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived the soft-switching three-phase power factor correction converter.